Ok, This may be broad but I need any starting points on painting a tall thin waterfall onto an existing image.   I generally render my water effects and touch them up in post.. however I simply don't have the render time required and need a quick dirty fix for a blury waterfall with frothing water at the base.

IE what brushes should I use, any brush settings, smudging.. I'm open to any suggestions.  Thanks.

Go to Google Image Search, and find some good waterfall reference. Paint the waterfall however you want, there's no special way needed.

Attached Link: http://www.renderosity.com/freestuff.ez?Topsectionid=11&Sectionid=83

I recently came across a tutorial for making a fake time-exposed shot of a river...maybe it might help you... Basically, create a layer that only cover the reigon where you want the waterfall. Filter>Clouds in that region, than filter noise, than filter motion blur with a 90 degree rotation (so it looks like its going up and down)then mess around with luquifying, perspective changing, etc to get the effect you want. Use the dodge tool to make highlights where froth should be...just something to try ;)

Other hints: In your paint program make a new layer to work on. Starting at your waterfall top, run a guide line for your 'fall'. If that line hits any protruding rocks, stop that portion, move the guideline out to the edge of that rock and start a new fall guideline. Degree of angle from vertical will depend on 'speed' or river's flow. For distant shots a straight guiding line will do, for close-ups, you may want to make the fall have more of a ballistic curve...use an elongated oval to create your guides.